The present invention relates to a laminate-type electrophotographic photoconductor which has an undercoating layer on an electrically conductive substrate and a photosensitive layer laminated on the undercoating layer. More specifically, the present invention relates to an electrophotographic photoconductor which has an improved undercoating layer and a method of manufacturing the improved photoconductor.
Electrophotographic photoconductors capable of rapidly producing high-resolution images are widely employed in copying machines, printers and facsimile machines. Many conventional photoconductors use photosensitive inorganic materials, such as selenium, selenium alloys, zinc oxide, and cadmium sulfide. Recently, photoconductors which use photosensitive organic materials have also been developed. These organic photoconductors provide the additional advantages of simple film formation, nontoxicity, light weight and other favorable characteristics. Among the organic photoconductors are the so-called laminate-type organic photoconductors, which include separate charge generation and charge transport layers. By properly choosing optimal material combinations for the layers, the sensitivity of the laminate-type organic photoconductor may be greatly improved. The laminate-type organic photoconductors also permit setting spectroscopic sensitivity at a desired wavelength of exposure light. Because of these and other features, laminate-type organic photoconductors have increasingly been used in electrophotographic apparatuses, such as copying machines, printers and facsimile machines.
Many of the laminate-type organic photoconductors now in practical use include a charge generation layer laminated on a conductive substrate and a charge transport layer laminated on the charge generation layer. The charge generation layer is formed by coating and drying a charge generation dispersion liquid onto a conductive substrate. The charge generation dispersion liquid consists of an organic solvent, into which an organic charge generating agent and a resin binder are dispersed. The charge transport layer is subsequently formed by coating and drying a charge transport dispersion liquid onto the charge generation layer. The charge transport dispersion liquid consists of an organic solvent into which an organic charge transport agent and a resin binder are dispersed.
The basic layer structure described above provides a photoconductor that exhibits the fundamental functions necessary for image formation. In practice, however, it is important to obtain high-quality, defect-free images, and it is also important to maintain high image quality after the photoconductor has been used repeatedly over long periods of time. To meet these requirements, the photosensitive layer must be homogeneous and free from defects. It is also important that the photoconductor exhibit excellent electrical properties. The film quality and electrical properties of the photoconductor should be stable enough to not deteriorate after the photoconductor has been used repeatedly over long periods of time.
The charge generation layer absorbs light to generate electric charge carriers, consisting of an electron and a hole. Since an electrostatic latent image should be formed on the surface of the photoconductor according to the applied field, the holes and the electrons must be quickly injected into the conductive substrate and the charge transport layer, respectively, before annihilation by recombination or trapping occurs in the charge generation layer. Therefore, the charge generation layer should be as thin as possible. The photoconductors now in practical use in electrophotographic apparatuses have charge generation layers as thin as several tenths of a micron or less. Since the charge generation layer must be such a thin film, dirt on the conductive substrate, nonuniformities in the shape and properties of the conductive substrate and surface roughness of the conductive substrate can result in a nonuniform charge generation layer. A nonuniform charge generation layer can result in image defects, such as voids, black spots and/or print density variations.
Usually, the conductive substrate is formed by drawing an aluminum alloy cylindrical tube. Alternatively, the conductive substrate may be formed by cutting and polishing the surface of an aluminum alloy cylindrical tube. In the aluminum alloy substrate, surface roughness differences, surface dirt, differences in the amount and size of the constituent metal precipitates, and uneven oxidation across the substrate surface may result in nonuniformity of the charge generation layer formed on the substrate surface. These nonuniformities in the charge generation layer may adversely affect the image quality.
To avoid nonuniformity in the charge generation layer, an intermediate layer (or undercoating layer) made of a resin with low electrical resistance is interposed between the substrate and the charge generation layer. The intermediate layer also creates a blocking effect that prevents hole injection from the conductive substrate, and thereby avoids diminution in the charge retention capability of the photoconductor.
Solvent-soluble polyamide, polyvinyl alcohol, poly(vinyl butyral), casein, and similar resins are used for the undercoating layer. A resin undercoating layer as thin as 0.1 .mu.m or less is sufficient to act only as a blocking layer. However, to smooth out variations in the surface profile and properties of the conductive substrate, to cover dirt on the conductive substrate, and to improve the wetness of the coating liquid for the charge generation layer and thereby avoid nonuniform formation of the charge generation layer, the undercoating layer should be as thick as 0.5 .mu.m or more. Depending on the preparation conditions of the substrate and the degree of contamination of the substrate surface, the undercoating layer may have to be as thick as 1 .mu.m or more. Such a thick resin layer made of the polyvinyl alcohol, solvent-soluble polyamide, or casein resins described above causes residual potential rise and change of the electrical properties of the photoconductor in extreme temperature and humidity environments. The residual potential rise and electrical property changes in turn cause image defects, such as residual images (called "memories") in a low temperature and low humidity environment, and minute black spots and voids in a high temperature and high humidity environment. The resins described above also absorb considerable amounts of water. Ion conduction by hydrogen ions and hydroxyl ions dissociated from the absorbed water are then responsible for most of the electrical conductance of the resins. Therefore, the electrical resistance of the resin layer could vary greatly, depending on the amount of water contained in the resin layer. This could produce further undesirable variations in image quality.
Various materials have been proposed to create a thick undercoating layer which has low electrical resistance, and which does not Vary with changes in the environmental conditions. The Japanese Unexamined Laid Open Patent Applications (hereinafter referred to as "JULOPA") Nos. H02-193152, H03-288157 and H04-31870 specify the chemical structure of a solvent-soluble polyamide resin. The Japanese Examined Patent Application (hereinafter referred to as "JEPA") No. H02-59458, and JULOPA Nos. H03-150572 and H02-53070 disclose additives that inhibit variations in the electrical resistance of the polyamide resin arising from changes in the environmental conditions. JULOPA Nos. H03-145652, H03-81778 and H02-281262 disclose mixtures of polyamide resin and other resins that allow adjustment of the electrical resistance of the resin, and thereby weaken the influence of environmental changes.
Other materials that may be used in place of polyamide resin include cellulose derivatives (JULOPA No. H02-238459), polyetherurethane (JULOPA Nos. H02-115858 and H02-280170), polyvinylpyrrolidone (JULOPA No. H02-105349) and polyglycolether (JULOPA No. H02-79859). Proposed crosslinked resins whose water content does not vary with changes in the environmental conditions include melamine resin (JULOPA No. H04-22966, and JEPA Nos. H04-31576 and H04-31577) and phenolic resin (JULOPA No. H03-48256).
However, the influences of temperature and humidity are unavoidable, so long as polyamide resins are used as a main constituent of the undercoating layer. Though the other materials mentioned above are effective if the resin layer is very thin, increases in the thickness of the resin layer cause increases in the electrical resistance of the photoconductor. Therefore, residual potential rise still occurs when the resin layer is as thick as several microns.